The source code is your best friend with VirtualTree...
TVTHintMode = (
hmDefault, // show the hint of the control
hmHint, // show node specific hint string returned by the application
hmHintAndDefault, // same as hmHint but show the control's hint if no node is concerned
hmTooltip // show the text of the node if it isn't already fully shown
);
I usually use these options to display the hint text of the node
ShowHint := True;
HintAnimation := hatFade;
HintMode := hmHint;
hmTooltip is using the text of the node so it may not call the OnGetHint in that case.